The response of sodium transport to hormones is studied in a cultured epithelium. Transport is assayed by measuring amiloride-sensitive sodium uptake into apical membrane vesicles prepared from A6 cells, a cultured cell line derived from toad kidney. Amiloride sensitive sodium channel will be examined for specific binding of 3H-labeled-Benzamyl. The hormones to be studied are aldosterone and antidiuretic hormone.